Islamist insurgents in Somalia have told the United Nations' World Food Program to stop importing food aid into the country.

The group al-Shabab said in a statement Wednesday that the massive importing of food is ruining Somalia's agriculture sector.

The rebels said the WFP must start buying food from local farmers for distribution to the needy.

Al-Shabab warned Somali businesses to stop working with the U.N. agency by January 1, and said the WFP must empty its warehouses of food aid by that date.

The United Nations estimates about half of Somalia's population -- about 3.8 million people -- is dependent on food aid. The world body blames the food shortage on drought and ongoing fighting between government forces and Islamist militants.

Al-Shabab controls much of southern Somalia.

Separately, the World Food Program and the humanitarian group World Vision have withdrawn staff from parts of southern Somalia because of violence.

A spokesman, Peter Smerdon, for the U.N. food agency tells VOA that staff members have been evacuated from the Jubba regions because of security concerns.

Al-Shabab is fighting to overthrow Somalia's moderate Islamist government and impose strict Sharia law throughout the country. The group has also fought clashes against another insurgent group, Hizbul Islam.